Transmission control circuits



1934- H. w. DUDLEY 1,971,679

TRANSMISSION CONTROL CIRCUITS ,Filed Jan. 30. 1931 2 Sheets-Sheet 1 FIG. a 1

A a sneer/vs V J I F62 9 k 7 A SELECTIVE V 7 c/ecu/rs 8 uvvmron H. W DUDLEY ATTORNEY Patented Aug. 28, 1934 GEFECE 1,971,679 TRANSMISSION CONTROL CIRCUITS Homer W. Dudley,

East Qrange, N. 5., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 30, 1931, Serial No. 512,246

2 Claims.

This invention relates to transmission control in signaling systems and particularly to circuits for reducing the effects of interfering waves in such systems.

An object of the invention is to discriminate between electrical waves of different characteristics, for example, between useful signals and interfering noise waves.

Another object is to improve the operation of signal-controlled devices for suppressing echoes and preventing singing in two-way signaling systems subject to interfering waves, such as static or line noise.

The invention in a broad aspect is a system for discriminating between speech waves or waves having energy characteristics similar to speech waves, and waves which over a definite frequency range have a substantially constant energy level, such as static or so-called line noise. A preferred embodiment of the invention may be used in connection with a two-way signaling system employing wave-controlled, anti-singing and echo suppression devices atthe terminals thereof. Discrimination between signals and interfering waves in such a system is obtained in part by utilizing, for selectively controlling certain of these devices at each terminal, a control wave preferably of a single frequency and fixed magnitude at one point in the system, transmitted from the other terminal along with the signals and under the control thereof. At the receiving station, the received control wave is eliminated from the signal receiving circuit to prevent it being heard by the listener, and at the transmitting station,

I the signal frequencies corresponding to those of the control waves are eliminated in the signal transmitting path so as to prevent false operation of the local and distant receiving control devices by them.

Further discriminationagainst static and noise is obtained in the system of the invention by the use therein of specially designed circuits for preventing operation of the wave-controlled antisinging and echo suppression devices by static or noise interference without decreasing the sensitivity of these devices as regards operation by the control waves or other signals. This circuit is designed to discriminate between the signal waves and the interfering waves, such as static or line noise, on the basis of the difference in the amount of power contained in small adjacent frequency bands of the two types of waves.

the case of static similar interference energy, the power in small adjacent frequency bands is substantially the same at any instant,

While in the case of speech waves or waves produced by a generator of a single frequency, such as the control waves employed in the two-way signaling system of the invention, a band width slightly less than that of the fundamental may or may not include a harmonic containing a substantially greater amount of power. These distinguishing differences are made use of in the discriminating circuit of the invention, which, specifically described, comprises one or more differentially wound relays, the windings of which are connected with the transmission path carrying the speech or control waves and the static or other noise waves through selective circuits, such as filters, each selecting a different small frequency band of the combined waves. The static or noise components of adjacent selected bands having substantially the same amount of power are balanced out in the differential windings of the relay, while the relay will be operated by the difference power of the speech or other signal components in the same selected bands, if only one contains a harmonic. Of course, if only two selective circuits are employed, and both the frequency bands selected thereby contain a har monic of the same magnitude, the speech or control wave components will be balanced out also. To get around this difficulty, it may be desirable to employ more than two selective circuits to insure that the speech or control Wave power is not present in equal magnitude in all of the selected bands. Three selective circuits should be sufiicient in which case three difierential relays, the operation of any one of which will give the required control of the transmission circuits, would be employed.

The invention will be better understood from the following detailed description thereof when read in connection with the accompanying drawings, in which Fig. shows diagrammatically a discriminating circuit in accordance with the invention in. its simplest form:

Fig. 2 shows diagrammatically a more complicated form of the discriminating circuit of the invention suitable for preventing false operation of the signal-controlled switching circuits by noise in a two-way submarine telephone cable or a radio telephone circuit; and

Figs. 3 and 4 show diagrammatically different forms of a radio telephone terminal circuit embodying the circuits of the invention.

The discriminating circuit of Fig. 1 comprises a normally open switch 1, which when closed controls over the circuit 2, the transmission efficiency of an electrical circuit (not shown) or the operation of apparatus therein in a desired manner. It is desired that the switch 1 be closed only in response to the signals, for example, speech signals, transmitted over the transmission 5 path 3, which path is also subject to interfering one contact 1a of switch 1, and when' theener r gizing current in the winding 6 of relay l predominates over that in the winding 5, the relay armature will make contact with the-contact 1b of the switch 1. As 1a and 1b are each connected to the other side of the icircuit 12, the switch 1 is closed to connect the two sides of the circuit '2 together when the energizing current in either windingofrelay 4 predominates over that in the other,

1' Thewmemg 510i the relay 4 is connected to the output of the wave-controlled, relay contro-lling device 'ljthe input'of which is connected through. the selective circuit 8 to theoutput Iof vthe: one-way wave amplifying device Q'theinput of which is connected across the line 3 carrying thersignals and subject to the interfering waves. Similarly, the opposing winding 6 of the relay 4 is .c'o rinec'ted so as to .be supplied with energizing current from thekoutput of the wave-controlled, relay-controlling device 10 the input of which is connected throughfthe selective circuit 11v also tothe output of the amplifyingdevice 9'.

Th e wave-controlled, relay-controlling devices 7 and '10 maybe vacuum tube amplifier-rectifier devices bf the ty e" well known in the art or any other devicejwhich will'respond to alternating current wavesiinpressed upon their inputs to producerectifiedvcurrentrof proportional amplitudes intheir outputs. The devices 7 and 10 should have the same amplification factor. i if i The selective circuits 8 and llimay be resonant circuits bf the type well known in the art, each tuned to a selected different frequency within" the frequency range ofthe waves impressed upon its inputgso as to transmit with little at'-. tenuation, the resonant and neighboring freqiiencie's in the impressed waves while producing appreciable attenuation'in other frequencies of the impressed waves. Alternatively, the selective circuits daud 11 may be band-pass filters of welliknown type, for example, such as disclosed in the United States patents to. Campbell, Nos. 1,227,113 and 1,227,114, issued may 22, 1917, in which case each filter isdesigned to transmit with little attenuation a different, narrow sub band of frequencieswitlnn the frequency range-ofgthewaves impressed upon its-input producing appreciable attenuationin the impressed frequencies outside the range of the selected subs-band. r The design oftheselectivecircults is :such? that: the frequency sub -bands transmitted withjlittle attenuation areclosely adja-i cent androf such;smal1 width that the aniount of energy'in each due to noise'or static is essen-- tially the same at any instant. 1 i

{The circuitof Figwl operates as follows: signals transmitted. oventhe transmission circuit 3 and theraccornpanyingnoise waves are applied simultaneously to" the input of the amplifying device r and are amplified thereby. The

amplified, :combined waves in the output of the amplifyin devices. :divide between th input; o

the selective circuit 8 and the input of the selective circuit 11. The selective circuit 8 selects from the impressed waves a frequency band of very small width compared to the total frequency range of the waves in the output of the device 9, and the selective circuit 11 selects from the impressed waves an adjacent frequency sub-band of the same width. c Y t The selectedsub-band inthe output of the selective circuit 8 is impressed on the input of .the device 7, which will produce in its output,

rectified currents of proportional amplitudes whichare supplied as energizing current to the winding 5 of the relay 4. Similarly, the small selected sub-band in the output of the selective circuit 11 is impressed on the input of the device 10 which will produce in its output circuit, rectiififid. currents ofproportional amplitudes which are supplied as energizing current to the winding 6 of the relay 4. The selective circuits 8 and 11 being designed so that the twofrequency bands transmitted 'therebyare closely adjacent in the frequency scale and .of sufficiently small width,'the' amount of power due to'nois'elor static in the selected bands supplied to the windings 5 and 60f relay 4 isflapproximately the sameand, therefore, .the'noise orstatic components are'effecti'vely balanced out in the relay winding. However, it is quite probable that the amount of power due to speech energy in the band in'the hutput of one .of' therrectifying devices? or 10 may hei'co'nsiderably larger than that in the'band in the output of the other device duelto the pres? ence therein of .one or more speech harmonics containing an appreciable amount :of power and the absence of a harmonic of equal magnitude inthe other hand. If that isthe case, the magni tude. of'the energizing current supplied to one of the windings 5 or 6 of the'reiay l will be sufficient ly' greaterthan that supplied to theother wind ing to. cause operation of the relay to attract its armatureso that it makes contact with either the cdn tact ldorithe-contact 1b of the switch 1 depending upon which'windingrof the relay has the largest energizing current, causing the circuit 2, to be short-circuited ,andthus controlling the transmission characteristicsof the circuit asso cia'ted'with circuit 2 or the apparatus therein in accordance with the speech wave'stransmitted over the circuit 3. I 1 r Ofchurse, there is a remote possibilitythat the two bands of waves selected by selective circuits 8 and ll may eachcontain a harmonic ofspeech of-the same magnitude so that the speech, will alsobe efiectively balanced out in the relay 4 in the same manner as the'noise'waves. This remote possibility may be prevented by modifying the circuit ofFig. asshown in-Fig. 2. As indicated in Fig. 2, an additicnalselective circuit lZ is connectedto the output of the am plifier El inparallelwith the selective circuits 8 and '11. free selectivecircuit 1'2,'which may be aband filter is designed to .transmit'with little attenuation from the impressedwaves-in theout put-of the amplifying device 9, a frequency band closely adjacent to that of the selective circuit 3 0 11 and-of the same width while producing appreciahle attenuation in the :impressed fre quencies outside therange of that selectedisub band;- A wave-pontrolled, 'relay-controlling' device 13, similar to thedevices {and it}, is ($911";

Whereas, in the system of Fig. 1, a single switch 1 controlled by a single differentially wound relay 4, was utilized for connecting the short-circuiting path across the circuit 2, in the modified system of Fig. 2, in addition to the switch 1, two other switches 14 and 15 controlled respectively by operation of the diiferential wound relays l6 and 17 are each adapted to produce a short circuit across the circuit 2 in parallel with that produced by operation of the switch 1. One of the diiferential, windings 18 or" the switch 16 is connected to the output of the device '7 so as to be supplied with energizing current from the output thereof. The other diiferential winding 19 of relay 16 is connected so as to be supplied with energizing current from the output of the device 13. One winding of the relay 17 is connected to the device 10 so as to be supplied with energizing current from the output thereof. The other differential winding 21 of the relay 17 is connected to the device 13 so as to be supplied with energizing current from the output thereof. The difierential windings 5 and 6 of the relay 4 are connected so as to be supplied with energizing current from the output of the device 7 and the device 10, respectively, as in the circuit of Fig; 1.

It will be seen that the connections are such that the rectified current in the output of the device '7, having a frequency range corresponding to that of the narrow frequency band selected by the selective circuit 8, will be divided between one winding 5 of relay 4 and one winding 18 of relay 16; that the rectified current in the output of the device 10, having a frequency ran e correspond ing to that of the different narrow frequency band selected by the selective circuit 11, will be divided between the other diiferential winding 6 of relay 4 and the winding 20 of relay 17; and

that the rectified current in the output of the device 13, having a frequency range corresponding to that of the diiT-erent narrow sub-band selected by the selective circuit 12, will be divided between the other winding 19 of relay 16 and the other winding 21 of relay 17.

As the frequency sub-bands selected by selec tive circuits 8, 11 and 12 have the same width, and the selected sub-bands are closely adjacent, the power in each is essentially the same at any instant. Therefore, the portion of the energizing current due to the noise components, supplied to each winding of each relay is approximately the same. As the two windings of each relay are wound in opposition, it is seen that the noise components are efiectively balanced out in the windings of each relay, and the operation of the switches 1, 14 and 15 is independent of the noise.

It is almost certain that one er more speech harmonics of appreciable power will be present in one of the frequency sub-bands selected by the selective circuits 8, 11 or 12, and that one of the other two bands will not contain a speech harmonic of equal magnitude to that in the first selected band. It is apparent, therefore, that the magnitude or" the energizing current in the two windings of at least one or" the relays l, 16 and 17 will be sufiiciently different to cause the armature of that relay to be thrown to one or the other of the contacts of the switch controlled ther by so as to close at least one of the short-circuiting paths across the circuit 2. The transmission efficiency of the associated transmission circuit or the operation of the apparatus therein will be controlled, therefore, by the signals transmitted over the transmission path 3. If the three selective circuits 8, 11 and 12 are properly designed, it is very improbable that the speech power will be present in equal magnitude in all of the selective circuits. However, this possibility may be made still more remote by employing a greater number of selective circuits and a corresponding number of relays and switches connected in a manner similar to that shown in the system of Fig. 2.

Fig. 3 shows the west terminal circuit of a radio telephone system. It comprises a transmitting circuit TC and a receiving circuit RC each associated in energy transmitting relation with a two-way telephone circuit TL and in conjugate relation with each other by means of the hybrid coil H and associated balanced network N in well known manner. Connected in the transmitting circuit TC between the hybrid coil H and the radio transmitter 31 including a transmitting antenna, is the one way amplifying device 32, the filter 33 and the delay circuit 34. Connected in the receiving circuits RC between the radio receiver 35 including a receiving antenna. and the hybrid coil H is the filter 36, the delay circuit 37 and the one-way amplifying device 38.

In the transmitting circuit TC between the hybrid coil H and the input of the one-way amplifying device 32 is a normally closed switch 39 adapted to be opened by operation of the mechanical relay 40. In the receiving circuit RC between the radio receiver 5 and the input of the filter 36 is a normally closed switch 41 adapted to be opened by operation of the mechanical relay 42. In the receiving circuit RC between the hybrid coil H and the output of the one-way amplifying device 38 is a normally open switch 43 adapted to be closed by operation of the mechanical relay 44.

Connected across the transmitting circuit TC between the delay circuit 34 and the filter 33 is a control circuit 45 comprising tl e wave-controlled, relay-controlling device 46 and the windings of the mechanical relays 4'? and 42 connected in parallel to the output of the device 46. A normally open switch 48 is adapted to be closed by operation of the relay 4'7 to connect the generator 49, of a control wave of the frequencies f1 which are preferably outside the speech frequency range, across the transmitting circuit TC r between the delay circuit 34 and the radio transmitter 31.

Connected across the receiving circuit RC between the normally closed switch 41 therein and the fi ter 36 is a control circuit 50 including the selective circuit 51, the wave-controlled, relaycontrolling device 52, and the windings of the mechanical relays 40 and 44 connected in parallel to the output of the device 52.

The east terminal circuit (not shown) of the i radio telephone system may be assumed to be identical with that at the west terminal circuit except perhaps for the value of the control wave frequencies, is, generated by the control wave generator there at the two stations, and the transmission frequency rangeof the filter 36 and the corresponding filter at the east terminal which depends upon the values of f2 and f1, respectively. The control wave frequencies f1 and f2 are both preferably outside the speech. frequency range. They may have the same or different values.

The selective circuit 51 in the control circuit 50 at the west terminal station maybe a resonant circuit of the type well known in the art, tuned to .a frequency; ,correspondingto ithe mid-free quencyof the band fz-generatedby the control wave generator ,atjtheseast-=terminal station so as to transmit :with'little attenuation; the resonant .frequency and neighboring frequencies in the impressed wave while producing appreciable attenuation inother frequencies of the impressed waves ,includinggthose in the 'speech frequency range; Alternativehnthe.selective circuit 51 may be a band-pass filter of well-knowntype; for example,- such asvidisclosed in United States patents to: Campbell, Nos: 1,227,113 and 1,227,114; issued May 22, 1917;;in' which-case the filter isidesigned to transmit with little attenuation the band of frequencies f2, while producing,appreciable attenuation at irequenciesroutside; that: band ine cluding the frequencieswithinIthespeech, frequency range. The selectivecircuit at the east terminal station corresponding to the circuit'5l atthe'west terminal station may have a design similar to'that of the latter circuit except that it Theoperation of: the system having terminal selcctsa band of frequencies f1 corresponding to that generated by the generator 49 at the-:west

terminal station: 1 i' x'The filter 36in thereceivingcircuit at the west terminal station may be a *band-passfilter .designed to suppress the frequencies fzgwhiletransmitting iefiiciently the. speech frequencies; The filter in the receiving circuit'a't the east; termi nal'statio'n. corresponding to .iilter 36 at thewest terminal station is also aband-pass filter but de signed to-suppress the-frequencies fl'J'Whi-ki trans,- mitting efficiently the 1 speechxwave frequencies. Thefilter 33in the transmitting circuit'at the west terminal station iand the corresponding filter in? the transmitting circuitatvth'e eastterminal station are also band pass filters bothdesigned to suppress'the frequencies fi'andjz while transmitting efiiciently other frequencies; 1 v

The-delay circuits 314 and 1 3'? may be of any type whichwill' produce adelay'of the required amount, which will bespecified .loelow, inthe transmission. of alternating current waves, *such as speech waves, therethrough. 'lTor example,

I each may be a low-pass filter; such as disclosed in "the Campbell patents mentionedabove, or a network such as disclosed for a similar pur-. p'oseinth'e United States patentto Arnold; No; l;565,302gissuedDecember 15'; 1925'. circuits corresponding tothat of Fig. 3 will now be described. I The speech waves receivedover .the line TL are 'impressed'by the hybrid coil'l-I upon theinput of the'transmitting' ciicuit'TCanda amplified by the amplifying device 32-therein; 'The amplified waves are transmitted through the filter 33 which will effectively suppress therefrom frequencies corresponding to -f1 and: f2 generated by the con trol wavegenerators at the west and east termi nal stations respectively. The passed waves' will divide between the' i'nput of the i delay' circuit iia in the-transmitting circuit TC and? the input of the control circuit 45. The portion" divertedinto the control circuit -will "operate the wave con':

trolled, relay-controlling device 16 therein causing the windings-of relays 42 and i'linits output to'loe energized: Relay' 42 thencperates'to open the normally closed"switch41in the receiving circuit'RC' makingthat'cir'cu --inoperativ e to transmit waves rrom the radio receiver '35 to the filter 36 and to the input of the control circuit 50. "Relay 4? operates simultaneously with relay 42' to" close the moment openswitch 548 so' as ,2; tocomiect the generator l9 to the input of the radio transmitter lcausing the control waves of rrequencies'f to be transmitted to the transmis: sionmedium. Meanwhile, the main portion of the. speechwaves in the transmitting circuit TC havebeen delayed .in-transmission, to the radiotransmitter Bibythe delay circuitS. The delayvcircuit Zia-is designedto introduce sufficient vdelay in1the:transmission of thev speech waves -t o insure thattheyare not impressed upon the 'radiortransmitter 31 before thegcontrolwavesh are transmitted thereto from generator 47.. The

, The;control wave f1 andthespeech wave transmitted by the radio transmitter at the west tern1inal-.-u 0n {arrival at the east terminal station will be picked up by the antennaof 'theradio receiver 35 thereat and then will be transmitted over the receiving circuit RC. The combined speech andicontrol waves'divide between the in put of the-control circuit 5'0 and the'input of the filter 36 in'the receiving circuit EC. The control waves ii are selected by: the selectiveicircuit. 51 inthe control circuit 50 and impressed upon the wave-controlled. relay-contro11in'gdevice '52. causing itsoperation' to-energiZe the windings or" relays-O'; and-44' inits output. Relay eflywill operate to open'the switch 39'; inthe input of the transmittingcircuit TC at the east terminal station thereby making that circuit inoperative t0 transmitfro'm'that'time speech waves from the circuitTL' to theinputiof the amplifying-device 32'. Relay 4 will operate simultaneously with relay 40? to close the normally open-switchS in receiving circuit RC, thereby making thatcire cuit operative to transmit waves from thezoutput of the. amplifying devicex38 therein to the. circuit'TL through the hybrid coil HZ. i Q'As thefilterGS' in the receiving circuit RC is designed to suppress frequenciescorresponding to the control wavefreque'nci'es f1, onlythe speech waves will be transmitted therethrough, and

throughrthedelay circuit 37 to the input of the amplifying-device 8'." The delay circuit '37';is designed to-prcduce'just suihcient delay in'the speech waves transmitted therethrough tolinsur'e thatthe switch 39 has been'closed under control of the control waves .71 aeefor'e the speech waves arriveithereat in the receiving circuit R0; The amplifiedz'speech waves inthe output or the amplifyingdevice 38",- the normallyopenswitch 43 having "been previously' closed by operation of the relay '44" in responseto' the control wave f1, will' be transmitted to thezcircuit TL" through theh'ybrid coill-Ip 1 1 The relays 46 44; should be designed to have sufficient. hangover intheir action, for ex-- ample,"by making them slow-to-releasarto main tain thein operated to hold switch 39-open"and switcheii "closed; respectively, for artime after the control device 52 releases due to acessation inz the supply of speech waves transmitted intocontrol circuit 50-, suificientizto prevent ;loss' of part of the speech or false operation of the transmitting control device. The corresponding relays at the east terminal station should be similarly designed.

From the above description, it is apparent that by the design of the filter 33 in the transmitting circuit T0 at the west terminal station and the corresponding filter at the east terminal sta-- tion to eliminate from the waves transmitted ther through, all frequencies corresponding to the frequencies of both control waves f1 and f2 generated at the west and east terminal stations, respectively, false operation of the transmitting and receiving control circuits by currents other than the control waves is prevented. The filter 36 in the receiving circuit R0 at the west terminal station and the corresponding filter 36 in the receiving station RC at the east terminal station are designed to eliminate frequencies corresponding to the control wave transmitted from the other terminal in order to prevent the control waves from being heard by the listener associated with the lines TL or TL, respectively.

False operation of the several switches in the transmitting and receiving circuits at the two terminal stations by line noise or static may be prevented by proper adjustment of the sensitivity of the relay control devices and/or the relays controlled thereby at each station in well known manner. It may be desirable to provide some means other than decreasing the sensitivity of the receiving control devices and/or the relays controlled thereby, which, of course, tend i to make the system more sluggish and unsatisfactory for high quality transmission, to prevent false operation of the switching devices by the static entering the receiving circuit along with the speech. The use of discriminating circuits of the invention such as illustrated in Fig. l

or 2 for this purpose will greatly improve the operation of the system as a whole.

If the circuit of Fig. 1 is utilized for this purpose, the relays 46 and 44 at the west terminal would each be provided with two opposing windings; the selective circuit 51 would be replaced by two selective circuits corresponding to the circuits 8 and 11 of Fig. 1, having their inputs connected through a suitable amplifying device across the receiving circuit BC in front of the filter 6, and the relay-controlling device 52 would be replaced by two similar devices corresponding to the devices '7 and 19 of Fig. l, the output of one of said devices being connected to one of the diilerential windings of each relay and the output of the other of said devices being connected to the other differential winding of each relay, and the inputs of the two devices being connected, respectively, to the outputs of the two selective circuits Each of the two selective circuits would be designed to pass one of two narrow frequency bands, one ranging from fz-A to f2+A and the other ranging from ,fz-l-A to fz+3A where A is a very small frequency range. If A is made sufiiciently small, the power in each selected band due to the static currents will be substantially equal, and the static, therefore, will be effectively balanced out in the windings of the differential relays. The control wave f2, however,

will not be balanced out in the relay windings,

since it is contained in one of the passed bands only. By utilizing three or more of the selective circuits and a corresponding number of the differentially wound relays connected in the manner shown in Fig. 2, the possibility of false operation by static may be made still more remote. A similar discriminating circuit operating in similar manner may be used at the east terminal station to insure operation of the switches 39 and 43' in response to the control waves f1 while preventing false operation of the switches by the static.

Fig. 4 shows the circuit of the invention applied to the two-way radio telephone terminal of the type in which the transmitting circuit is normally disabled, such as is used in the present transatlantic radio telephone system. It differs essentially from the circuit shown in Fig. 3 only in that an additional relay 53 is connected to the output of the wave-controlled, relay-controlling device 46 in parallel with the relays 12 and 47, the additional relay 53 operating in response to operation of the device 46 by the waves diverted into the control circuit 4.5 from the transmitting circuit TC to close the normally open switch 54 in the transmitting circuit TC bet" can the point of connection of the control wave generator 49 thereto and the radio transmitter 31. In this circuit the normally disabled transmitting circuit makes it unnecessary to have the transmitting and receiving antenna located at widely separated points to reduce transmission therebetween to a sufiiciently low level as to produce no ill effects on th received speech or the wave-controlled receiving switching devices as would be required in the system of Fig. 3.

The use of an auxiliary control wave instead of the speech waves themselves for controlling the switching apparatus in a two-way signal transmission system as in the circuits of the invention shown in Figs. 3 and 4, constitutes a decided improvement from the standpoint of an increase in discrimination against static because of the smaller frequency band which is utilized for control. It is estimated that this discrimination would amount to as much as 20 decibels due to the use of the smaller frequency band, and that the discrimination against static would be increased by another 20 decibels by using at each terminal station, a receiving control such as shown in applicants Figs. 1 and 2. In addition there is an appreciable gain due to the control currents being of much more nearly constant magnitude than is the speech current. These gains would seem to be sufficent to insure satisfactory operation even though the static power were considerably greater than the speech power. As the transmitting-control device 46 associated with the transmitting circuit TC at the west terminal station and the corresponding device at the east terminal station in the system of Fig. 3 or Fig. 4-. receives only the speech and land line noise, it can be worked at greater sensitivity than the receiving side which is subject to static so that less transmitting delay is needed to be provided by the delay circuit 34 or 3-1. This results in shorter hangover times and, therefore, shorter breaking times. The control current may be utilized for other purposes in addition to that of controlling the switching apparatus. For example, it may be utilized to indicate the transmission equivalent of the radio path if a meter with a suitable scale is utilized in the receiving control circuit 50 or 50.

It is apparent that the principles of the invention may be applied to other circuits diifering considerably from the circuits disclosed without departing from the spirit and scope of the invention'. The invention is only to belimited cordance with the appended claims.

What-is claimed is: r l 1. A signal receiver subjected to combined Waves including signal waves of energy characteristics similar to speech and interfering waves of substantially constant energy level over adefinite frequency range, comprisingmeans for subdividing the frequency band of the combination waves-into aplurality of sub-bands'within said definite frequency range, means for balancing said sub-bands againsteach other so as to effectively neutralize the energy therein due to said interfering waves, and means'respensive' todifferences in the energy level in said sub-bands balanced against each other caused by the presence in one or more of a signal harmonic and the absence of asignal harmonic ofequal power content one ormore of the other sub-bands, fer distinguishing between said signal waves and said interfering waves. 7 1

in ac- 29A signal receiver subject to combined alternating current waves including signal wavesof energy characteristic similar to speech and interfering waves of substantially constant energy level over a definite frequency range, comprising means for subdividing the frequency band'of the received combined waves into three or more frequency sub-bands within'said definite frequency range, three or more mechanical relays each having two'opposing windings, means forrectifying each of the subdivided frequency'bands, and for applying therectified bands to the windings of said relays so thatthe several bands are-balanced against each other inthe relays, and receiving means responsive to a difference between the amplitude levels of-the current in the two windings of any one of the relays.

HOMER W. DUDLEY. 

